This invention relates to a highly corrosion resistant plated steel strip or sheet and manufacturing method thereof and, more particularly to a Fe-Mn plated steel strip intended for use in automobiles with its excellent phosphate treatability, adhesion and corrosion resistance after painting, and manufacturing method thereof.
For steel strips intended for use in automobiles, the plating layer showing excellent phosphate treatability, adhesion and corrosion resistance after painting is typically required.
The process for making a corrosion resistant plated steel sheet are disclosed by GB 2140035A and EP 0125658. GB 2140035A discloses an iron-zinc alloy electro-galvanized steel sheet having a plurality of iron-zinc alloy coatings comprising; a lower layer formed on the surface of a steel sheet; and an upper layer comprising at least two iron-zinc alloy coatings formed on the lower layer. The iron content of each coating in the lower layer is from 1-15 wt % and the total coating weight of the lower layer is from 1 to 50 g/m.sup.2. And the iron content of each of the coatings in the upper layer is over 15 wt % and the total coating weight of the upper layer is from 1-40 g/m.sup.2.
EP 0125658 discloses a steel strip having a layer of Fe-P alloy with a phosphorus content of from 0.0003 to 15% by weight electrodeposited on at least one surface of the steel strip to build-up of at least 0.01 g/m.sup.2 of Fe-P alloy on the underlying layer of zinc or zinc alloy.
The above-mentioned prior arts involves the following problems;
The iron-zinc alloy electro-galvanized steel sheet disclosed in GB 2140035A exhibit inferior corrosion resistance, because moisture penetrated throught cutting or scratch reacts with zinc to form powder-type corrosion product of zinc hydroxide that further allows water to penetrate.
And the Fe-P plated steel strip disclosed in EP 0125658 is revealed to have the peeling off problem in the upper plated layer, that is caused by increase of brittleness due to phosphorus and also by poor adhesion due to small amount of Fe or P segregated during early period of plating process forming the upper layer via unnecessary non-electrolytic reaction between zinc ions contained in the lower plated layer and Fe or P ions in plating bath.